How large is just their body in comparison to a coin? Such as, Nickel size? Quarter size?
There is no way to tell for sure until they actually drop down their breeding tubes to spawn.
Sometimes they'll get along fine UNTIL they reach adulthood and are ready to spawn.
Pre-spawning behavior usually begins with the two going around eating off the same leaf together. They will do everything together. Sometimes liplocking also begins. But you'll know for sure if you see them begin to clean a certain leaf off together, or the slate (if you have one for them). Sometimes they'll spawn even on the intake tube of the filter if you don't provide slate for them.